1. Field of the Invention
The present invention relates generally to non-volatile semiconductor memory devices and, more particularly, to methods for reliably operating non-volatile memory devices.
2. Description of Related Art
Non-volatile memory (NVM) devices are in widespread use in a variety of industrial, commercial, and consumer products. Cell phones, personal data assistant (PDA) devices and modems are just a few examples that include embedded NVM modules in their normal configuration.
Integrity of data stored in NVM modules is maintained even when power to the memory is lost. For example, when a cell phone is turned off, names and telephone numbers of a user's phonebook normally remain stored in NVM of the cell phone. Unlike other forms of memory such as masked read-only memory (MROM), which normally is programmed at time of manufacture and which programming cannot thereafter be changed without an expensive change to a mask that defines contents of the MROM, data stored in NVM can be modified by a device in which the NVM is embedded. For example, a user can modify stored values of names, telephone numbers, and other data stored in the NVM of a cell phone. Such storing requires that relatively complicated operations be performed that, while not visible to a human user, are essential to maintaining integrity of stored data.
It typically is a responsibility of a user of an NVM device (e.g., a cell phone manufacturer) to operate the NVM and/or any associated memory controller according to specifications provided by a manufacturer of the NVM. Actions may nonetheless be performed by the user that violate manufacturer's specifications. A memory controller can detect such violations by comparing a value of an applied operating voltage, such as Vcc, with a high limit, HVCC, and a low limit, LVCC, and can inhibit any attempt to write to the NVM when the value of Vcc falls outside the HVCC/LVCC limits. However, process variations and temperature effects may cause the LVCC and HVCC detectors to be inaccurate, resulting in a failure of the memory controller to read/write a correct value from/to non-volatile memory when specified operating voltage limits are violated.
A need thus exists in the prior art for a method of preventing a read/write operation from/to portions of a non-volatile memory when reliably of the operation is in doubt.